Physiological sensor systems, such as pulse, tissue and cerebral oximeters, are used to measure a variety of physiological characteristics in body tissues using two wavelengths of light. The system generally includes a monitoring system connected to a sensor pad that adheres to the portion of the body being tested. The sensor pad includes a plurality of light sources in optical communication with at least one light detector. The light sources are activated by applying an excitation current source to an electrical wire connected to each light source. When activated, the light sources transmit light at specific wavelengths through the body tissue to the optical detector. The amount of light received by the light detector after attenuation by the body tissue is indicative of the physiological characteristic being tested.
To improve the accuracy of the measurement, or to enable the measurement of additional physiological characteristics, additional wavelengths of light can be used. This generally necessitates the addition of light sources requiring additional wires to carry the excitation potentials. Unfortunately, the addition of wires adds to the cost and complexity of the system. Moreover, monitoring systems are generally configured to work with sensor pads having a fixed number of wires. For example, if a monitoring system is configured to work with sensor pads having a three wire configuration, a sensor pad using additional light sources and having any more than three wires may not be compatible with the existing monitoring system.
One known method used to minimize the number of wires in a sensor pad when increasing the number of light sources includes having multiple light sources connected in a matrix of rows and columns of wires. The light sources in this configuration are activated by sequentially addressing the row and column of each light source with an excitation path. In this way, four wires provide connection and activation of four light sources. If pairs of light sources are connected in parallel, the same configuration of four wires can be used to connect and activate up to eight light sources. This configuration, however, requires a minimum of four wires and is limited to a maximum of eight light sources.
Accordingly, the embodiments described hereinafter were developed in light of these and other drawbacks associated with increasing the number of light sources in a physiological sensor without increasing the number of wires.